1. Field of the Invention
The present invention relates to a trench buriedbit line mask ROM process, and more particularly to one which forms a U-shaped channel preventing unwanted ion diffusion into bit line regions of the ROM structure.
2. Description of the Prior Art
A buried-bit line mask ROM is by far the most competitive ROM structure and the top view thereof is illustrated in FIGS. 1 and 2. In this example, a plurality of spaced word lines W/L (here only two are shown) are formed above a plurality of parallel bit lines B/L (here only two are shown) and both are separated by a nonconductive oxide layer OX. FIG. 2 is a cross-sectional view taken along a line 2--2 of FIG. 1, where a region 8 between the two adjacent bit lines B/L is defined as a channel region of a memory access transistor. Two spaced photoresist strips PR are separately formed at the top of the word line region W/L and constitute a programming window 7 therebetween. More specifically, the window 7, as highlighted with bold lines in FIG. 1, is defined above each word line region W/L which is between two adjacent bit lines B/L and substantially above the channel region 8. To block the channel region 8, i.e., to turn the memory transistor into having a relatively high threshold voltage (V.sub.T) in ROM code programming, boronions B+ are implanted from the window 7, tunneling through the word line area W/L, the oxide layer OX, and doped into the channel region 8, thus increasing the threshold voltage (V.sub.T) of the memory transistor, i.e., blocking the channel even for a certain word line W/L or gate voltage conduction on the transistor. However in programming the ROM, some Boron ions B+ will be diffused into the two bit line regions B/L adjacent to two sides of the channel region 8 thus resulting counter-doping effect in the bit lines B/L. This increases the resistance and the capacitance of the bit lines B/L and also decreases the junction breakdown voltage thereof thus negatively affecting the function of the ROM. More specifically, high bit-line resistance and capacitance hurt the product speed of the high density ROMs.
It is noted that each cell size in the ROM structure is mainly determined by the word line pitch and buried-bit line pitch. When the cell dimension shrinks for high density application, the buried-bit line doping needs to be reduced in order to avoid the bit-line to bit-line punch-through. This N+ doping becomes more sensitive to the counter-doping from P-type (Boron) doping for programming in the channel region.
It is requisite to provide a new method for fabricating trench buried-bit line mask ROM to solve the problems as mentioned previously.